5. Suppose the data concerning first-year salaries of graduates follows normal distribution with the population mean ? = $60, 000 and the stan- dard deviation ? = $10,000. Denote X as the salary of a randomly selected graduate.

(a)

What is the probability that X is greater than $40, 000?

(b)

What is the probability that X is between $30, 000 and $90, 000?

(c)

Suppose we interviewed 5 randomly selected SBU graduates

and their salaries are X1, X2, …, X5. Denote the average salaries

of these 5 people as X = X1+X2+X3+X4+X5 . What is the ex-5

pectation and the variance of X?

(Hint: Use the standard normal table to answer (a) and (b).)